
Hello Lisa, you have given only a very little bit of information about your dog and your circumstances. My reply is based only on what you have said.
A 15 year old dog is very elderly indeed, and to get one to that kind of age you have obviously looked after her very well. She has relied upon you to provide everything she needs, and make the right decisions about her care. She now needs you to help her. It sounds to me that life doesn't hold much fun or pleasure for your girl, and to wear a diaper means she has lost her dignity.
Lisa, let her go. Give her sleep, hold her and cry for her while she goes. Don't let her suffer further. Let her go.
Thinking of you both, bless you.
Dawn R.